I don't think you understand what main and universe are. They are not packages, but lists of available packages.

Running sudo apt-get update downloads these lists of packages, but you have to run sudo apt-get upgrade in order to actually install the available updates.

Therefore it is vitally important to always update your main and universe package lists. However the downloads should not be 13MB - your question is effectively a duplicate of this one: the size of apt-get update lists is too big
.